The broad goals of this research are to understand the regulation of extrachromosomal gene expression as related to toxin synthesis in bacteria, and the molecular mechanisms of membrane-directed toxicity. The virulence of numerous disease causing organisms depends upon toxins encoded by extrachromosomal elements. Furthermore, many toxins are known to affect the structural and functional integrity of cell membranes. However, the mechanisms of toxin activity remain unknown for many such toxins. Some strains of paramecia harbor endosymbiotic bacteria which confer upon their hosts the ability to kill related endosymbiontic-free paramecia. The killer phenomenon is dependent upon the extrachromosomal elements (plasmids and bacteriophages) of these endosymbionts, for these extrachromosomal elements direct the synthesis of a toxin whose action disrupts membrane functions of the so-called sensitive paramecia. In addition to the toxin, a proteinaceous inclusion body (R body), also necessary for killing, is encoded by the extrachromosomal elements. An understanding of killer paramecia will extend to other virulence systems. Thus the long-term goals of this research are to establish the genetic and physiological basis of the killer phenomenon among paramecia. Specific aims include: (1) cloning and expressing R body and toxin genes from specific species of endosymbiotic bacteria, and (2) testing membrane components for their ability to bind toxin.